Ion implanters are widely used in semiconductor manufacturing to selectively alter the conductivity of materials. In a typical ion implanter, ions generated from an ion source are transported downstream through a series of beamline components which may include one or more analyzer and/or corrector magnets and a plurality of electrodes. The analyzer magnets may be used to select desired ion species and filter out contaminant species or ions having undesirable energies. The corrector magnets may be used to manipulate the shape of the ion beam or otherwise adjust the ion beam quality before it reaches a target wafer. Suitably shaped electrodes can be used to modify the energy and the shape of the ion beam. After the ion beam has been transported through the series of beamline components, it may be directed into an end station to perform ion implantation.
FIG. 1 depicts a conventional ion implanter system 100. As is typical for most ion implanters, the system 100 is housed in a high-vacuum environment. The ion implanter system 100 may comprise an ion source 102 and a series of beamline components through which an ion beam 10 passes. The series of beamline components may include, for example, an extraction manipulator 104, a filter magnet 106, an acceleration or deceleration column 108, an analyzer magnet 110, a rotating mass slit 112, a scanner 114, and a corrector magnet 116. Much like a series of optical lenses that manipulate a light beam, the ion implanter components can filter and focus the ion beam 10 before steering it towards a target wafer 118.
As the semiconductor industry keeps reducing feature sizes of electronic devices, ion beams with lower energies are desirable in order to achieve shallow dopant profiles and shallow p-n junctions. Meanwhile, it is also desirable to maintain a relatively high beam current in order to achieve a reasonable production throughput. Such low-energy, high-current ion beams may be difficult to transport within typical ion implanters due to limitations arising from space charge. To prevent “blow-up” of a positive ion beam, negatively charged particles, such as electrons or negative ions, may be introduced for space charge neutralization. One way of sustaining space charge neutralization is through magnetic confinement of negatively charged particles. However, existing magnetic confinement approaches tend to introduce extra magnetic field components that cause ion beam distortion.
For example, FIG. 2 illustrates a conventional method for confining electrons with permanent magnets 202. The permanent magnets 202 may be arranged into two banks, one above and the other below a beam path associated with an ion beam 20. Taking advantage of electrons' tendency to cling to and spiral around magnetic field lines, the permanent magnets 202 can confine electrons (or other charged particles) in cusp magnetic fields in or near the beam path. Generally, the magnet field strength produced by the permanent magnets 202 should be weak enough not to affect transport of the ion beam 20. Note, however, the permanent magnets 202 in existing magnetic confinement approaches are typically arranged for “polar symmetry,” wherein like poles face each other across the beam path. That is, the north pole of a magnet in one bank faces the north pole of a corresponding magnet in the other bank. The same is true with south poles. The polar-symmetric arrangement of the permanent magnets 202 may produce a non-zero magnetic field component (Bz) in the mid-plane between the two banks of permanent magnets 202. The non-zero magnetic field component Bz can cause any part of the ion beam 20 that is not traveling strictly along the Z-direction to be deflected in the vertical (±Y) directions, resulting in vertical asymmetries in the ion beam 20. Such vertical asymmetries are typically difficult to correct with other beamline components.
In view of the foregoing, it would be desirable to provide a technique for confining electrons in an ion implanter which overcomes the above-described inadequacies and shortcomings.